Beat peanut butter and butter in medium bowl with electric mixer until smooth. On low speed, gradually beat in confectioners' sugar and 3 to 4 Tbsp milk until blended. Beat mixture on high speed 1 minute or until fluffy.

Spoon filling into a pastry bag fitted with a 1/4-in. plain round tip. Insert tip into the top of each cupcake; squeeze as much filling into the center of each as possible (top of cupcake will bulge slightly).

Melt chocolate chips in large glass bowl in microwave as package directs. Add sour cream; beat with an electric mixer until fluffy and good spreading consistency. Spread about 2 Tbsp frosting on each cupcake.

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